These extensions will work if they use native Joomla database layer (JFactory::getDbo()).

MightySites doesn't change registration process, it's up to you how to display any sites selector.
If all sites use same users database tables - all users will be already same on all sites, no sense to any sites.

If each site uses own users tables and ask user to select sits to register - your custom code should add user to all tables. But note that such users won't be synced.

Yeah, we want the users to be registered on all sites, but not necessarily show up in search of maps or such. Sounds like we need a separate registration field for "active sites" which we check for when displaying search results, something like that?

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